1. Field of the Invention
This invention relates to a process for producing an acrylamide polymer having a high molecular weight containing only in a small amount the water-insoluble matters formed in the process of producing the polymer, particularly in the step of drying the polymer. More particularly, it relates to a process for obtaining a dried product of said polymer, which has a high molecular weight and is good in dissolvability in water, by polymerizing acrylamide alone or a monomer mixture comprising acrylamide and at least one monomer copolymerizable therewith in an aqueous medium and then drying the resulting hydrous polymer, characterized by allowing a specific phenol, thioether or phosphite to be present in the system in said polymerization step or in said drying step.
2. Description of the Prior Art
In recent years, water-soluble acrylamide polymers have come to be used in large amount in many fields, for example, as paper-strengthening agents, thickeners for paper-making, reagents for recovering petroleum, flocculants, and the like. Among these uses, thickeners for paper-making, flocculants and the like, in particular, require that the polymer has a very high molecular weight, and those having an average molecular weight of ten million or more are recently not rare.
Some methods for obtaining such an acrylamide polymer having a high molecular weight have been proposed, however, industrially, the polymerization is often effected with a free radical initiator in an aqueous medium. In this case, the hydrous polymer obtained usually contains 60-80% of water and is in the form of an aqueous solution, but it is a rubber-like substance which has substantially no fluidity or a viscous liquid difficult to flow because the molecular weight of the polymer is very high. Accordingly, the hydrous polymer as such is difficult to handle and uneconomical in transportation, and also it has the disadvantage that the rate of dissolution of the polymer in water when using the hydrous polymer is very low. Therefore, the aforesaid hydrous polymer is usually formed into a dried powder by removing water therefrom by some methods, and one of the methods of removing water is a method of heat-drying the hydrous polymer as obtained with hot air, or the like. This method is often industrially used because it is simple in principle and has many advantages even in respect of production. However, acrylamide polymers tend to be crosslinked between their molecules upon heating, and when this is caused between very high molecular weight molecules, even though a very slight amount of crosslinkage is formed, the dissolvability of the polymer in water is decreased.
When the decrease in dissolvability is slight, it can be compensated for by such a means as to prolong the dissolving time when the dried polymer is used. However, when the decrease is great, the polymer is only swollen even when stirred in water for a long time, and gives a solution containing many insoluble particles. Therefore, when said solution is used for treating waste water or the like as a flocculant only a low flocculating ability is shown, and when it is used as a thickener for paper-making, fish eyes are formed on the web formed. Accordingly, when such a high molecular weight hydrous polymer is dried, there is adopted, for example, a method of drying under reduced pressure or blowing air at a relatively low temperature of 60.degree. C. or less.
However, it is desirable that the polymer is dried rapid at a temperature as high as possible, in respect of productivity, and, for this purpose, there have been proposed many agents for preventing insolubilization upon drying. However, it has been a common opinion that the formation of insoluble acrylamide polymers results from the formation of imide linkages between the polymer chains. Among the above proposals, it can be said that there is no case in which any insolubilization preventing measure is taken in addition to the prevention of the imide linkages from being formed.